1. Field of the Invention
The invention relates to a heat exchanger having a vacuum tube with an outer wall, and having an inner tube which holds a fluid and whose outer wall is arranged concentrically to the outer wall of the vacuum tube.
2. Description of Related Art
The heat exchanger having a vacuum tube is today the most important component for converting solar energy into heat energy in solar heating technology.
Such a heat exchanger is known, for example, from U.S. Pat. No. 4,186,724. The inner tube for holding the fluid is composed of two concentric tubes in which the fluid can flow in the counter-current mode. This inner tube is surrounded by an insulation space which is bounded at a distance from the fluid by the inner wall of the vacuum tube. The inner wall and the outer wall of the vacuum tube are arranged concentrically and form in cross section a ring, under a partial vacuum, around the inner tube.
U.S. Pat. No. 4,307,712 discloses a further heat exchanger of this kind, in which the inner tube is connected, as far as possible in an integral fashion, to transverse ribs in order to conduct heat better.
In the known heat exchangers, various reflection surfaces are inserted or various elements are blackened. The pipes of the fluid-conducting system are usually composed of a material which is a good conductor of heat. However, no elements are known which are provided for a good transfer of heat from such reflection surfaces to the fluid-conducting pipe system. The ribs which are known from U.S. Pat. No. 4,307,712 are complicated to connect to the fluid-conducting pipe system and do not have any connection to the partial vacuum tubes.